1. Field of the Invention
The present invention relates to a threaded fastener or a drill screw in which a threaded shank is formed at its forward end with a drilling portion.
2. Description of the Related Art
A drill screw of the type mentioned initially, advantageously drills its own pilot hole and then, cut threads in the work for drilling purposes.
The drill screw may be formed by two basic processes, one is by a machining process, for example, fabricating a drilling portion of the drill screw by the use of a milling cutter, and the other is by a pinching or cold heading and rolling process. The latter process is advantageous over the former in productive and economical senses.
A conventional drill screw generally includes a drilling portion having a body extending axially from a shank and a drilling tip extending forwardly of the body. A pair of diametrically opposite axially extending flutes are formed in the drilling portion to form surfaces which terminate at cutting edges. The cutting edges include inclined cutting edges located at opposite side edges of the drilling tip, and side cutting edges joined angularly to the inclined cutting edges and located at opposite sides of the body. Like a general-purpose drill, a web (or core) is provided between the flutes and extends from the center of the body to the forward end of the drilling tip. To this end, the inclined cutting edges are oppositely offset from a diametral axial plane of the drilling tip and lie in a plane parallel thereto. The drilling tip has inclined surfaces or body clearances extending rearwardly and angularly from the tip and thus, a chisel point is defined by a ridge where the two inclined cutting edges are joined.
Like a general-purpose drill, in a conventional drill screw, such a web is formed at the forward end of the drilling tip to form therein a chisel point.
However, such arrangement makes it difficult to insert the drill screw into the work during the starting of the drilling operation. That is, such a chisel point is defined by the joining of oppositely disposed body clearances. Accordingly, the chisel point is of V-shape in section and two surfaces are oppositely and symmetrically inclined with respect to chisel edges. One of the surfaces is a "rake face" and the other is a "relief surface." When the drill screw is rotated for insertion into the work, the "rake face" is inclined relative to the surface of the work or the horizontal plane at an angle of less than 90.degree. or at a "negative rake angle." It is for these reasons that the chisel point forcibly drills the work under pressure and friction and undersirable drilling results during the starting of the drilling operation.
This is due to the fact that conventional drill screws are designed in the same manner as general-purpose drills.
As to the general-purpose drills, a web is essential to maintain its strength as well as to always keep its cutting edges sharp by abrasion. On the other hand, a drill screw is used only once and therefore, sharpening by abrasion need not be made. Such a web may be used only to maintain the strength of the drilling portion.
In order to improve operating characteristics during the starting of the drilling operation, it is desired that end cutting edges of the drilling portion are made as sharp as possible. To this end, thinning has hitherto been effected to sharpen the forward end of the web. However, the chisel point still remains at the forward end of the web regardless of such thinning and a "negative rake angle" is formed therein. The problem thus remains unsolved.
In order to solve the foregoing problem, instead of the chisel point, it is necessary to provide a "positive rake angle", namely, angle formed between the work surface and the rake face is greater than 90.degree.. Importantly, in case that the drilling portion is made by cold heading and rolling techniques by the use of a pair of dies, it is impossible to provide end cutting edges having a "positive rake angle" at the web while the web is still presented at the forward end of the drilling tip. Therefore, only end cutting edges are to be provided while eliminating the web. In this manner, the "positive rake angle" can be defined if the drilling portion is made by cold heading and rolling techniques. As mentioned earlier, the drill screw, unlike the general-purpose drill which is used normally for a prolonged period of time, is used only once and therefore, no problem arises if the web is not provided.